1. Field of the Invention
The present invention relates to a display panel control device, a liquid crystal display device, and the like. More specifically, the present invention relates to backlight drive of the liquid crystal display device.
2. Description of the Related Art
In order to improve motion blur generated in moving pictures that are displayed in a hold-type display device such as a liquid crystal display device, there has been proposed for a case of a liquid crystal display panel, for example, to make the display to be of an impulse type in a pseudo manner, such as employing black insertion drive which displays a black signal between image signals at a specific rate, blinking backlight drive which controls a back-face light source (referred to as a backlight hereinafter) of the liquid crystal display panel to blink, and scanning backlight drive (Japanese Unexamined Patent Publication 11-202289: Patent Document 1), etc.
FIG. 47-FIG. 52 show luminance waveforms of a given point on a screen of each drive type (normal drive, black insertion drive, scanning backlight drive) for white display and black display, respectively.
As shown in FIG. 47-FIG. 49, in a case of all-white display, white luminance of the black insertion drive is reduced to a half level with the normal drive since human beings recognize the average luminance of the white display and the inserted black display. Further, as shown in FIG. 50-FIG. 52, in a case of black display, the luminance waveform thereof becomes the same half level as the black display with the normal drive. As a result, the contrast is also reduced to a half compared to that of the normal drive.
Furthermore, as described above, the luminance of white display is reduced to a half, and the backlight that takes up most of the consumed power is lighted up constantly even in a period of black display. Thus, the luminance efficiency goes down, so that the power consumption becomes increased for obtaining the luminance that is equivalent to the luminance before employing the black insertion drive. Therefore, it is difficult to keep the balance of overall performance.
That is, with a method using the black insertion drive, the display luminance in the period other than black display is decreased instead of improving the motion blur in the moving pictures. Thus, the contrast in the display becomes deteriorated. Further, the backlight is always lighted up even in the period of the black display, which results in having poor luminance efficiency. Therefore, the power consumed therein needs to be doubled for having the luminance that is equivalent to the luminance before employing the black insertion drive.
In the meantime, as shown in FIG. 53 and FIG. 54, in the case of all-white display on the liquid crystal display panel, white luminance is reduced to a half with the blinking backlight drive which lights up/off the back-face light source sequentially in accordance with scanning lines of the liquid crystal display panel than the normal drive since human beings recognize the average luminance of the white display and the inserted black display. However, the black display is also reduced to a half as in the white display since the backlight is turned off. As a result, the contrast becomes equivalent as the case of the normal drive. Further, while the luminance of the white display is reduced to a half during the black display, the power consumption is also reduced to a half since the backlight is turned off. Therefore, the luminance efficiency becomes equivalent as the case of the normal display. Thus, this method seems to be an excellent method.
That is, the blinking backlight drive seems like an excellent method having no deterioration in the luminance efficiency when considering a given single pixel alone, since it can improve the motion blur in the moving pictures, reduce the luminance of the black display as in the case of white display, have no deterioration in the contrast, and decrease the power consumption during a period where the backlight is turned off.
However, the liquid crystal display panel shown in FIG. 53 and FIG. 54 employs line sequential scanning. Thus, the time at which frame information is rewritten to each line differs, and levels of reaching the liquid crystal response in each scanning line of the liquid crystal display panel varies in a period where the backlight is lighted up. Thus, there is difference (bright and dark) in the luminance depending on the positions on the screen. Further, when the light-up timing of the backlight comes to go over the line sequential scanning, the moving pictures are to become shifted greatly.
In order to overcome the issues of the black insertion drive and the blinking backlight drive described above, there have been proposed following Patent Document 1, Patent Document 2, and Patent Document 3, for example.
As shown in FIG. 73, Japanese Unexamined Patent Publication 11-202286 (Patent Document 1) proposes scanning backlight drive that uses an illumination part 1001 for illuminating a liquid crystal display part, which has a plurality of light emitting areas 1002 that emit light to a scanning direction, and the plurality of light emitting areas 1002 are sequentially scanned and lighted up by synchronizing with a vertical synchronous signal of the liquid crystal display part.
With this scanning backlight drive, the divided light-source blocks are sequentially lighted up and off by synchronizing with the line sequential scanning of the liquid crystal part. Thus, the difference in the reaching levels of the liquid crystal response of each scanning line becomes smaller than the case of the blinking backlight drive described earlier.
In the structure of a backlight of Japanese Unexamined Patent Publication 2007-66634 (Patent Document 2), as shown in FIG. 74, a light emitting part 1020 having a plurality of light emitting areas 1021 that emit light at different timings is being divided into the plurality of light emitting areas 1021 by being in contact with a light transmitting device 1025 with partition plates 1023 (at least a part of which is formed by a light transmitting material). A reflection film 1022 pasted on the partition plate 1023 is provided to reflect the light from an emission light source 1024 of the respective light emitting area 1021 to have it emitted to the directions of the light transmitting device 1025 and a liquid crystal panel 026.
A display control circuit of Japanese Unexamined Patent Publication 2006-99100 (Patent Document 3) includes: a vertical timing control circuit which generates start signals STHA and STHB; a gate driver and a source driver which sequentially drives a plurality of OCB liquid crystal pixels by each row through controlling the signal STHA to keep gradation display pixel voltage to the pixels on the driven row, and sequentially drives the pixels at least by each row through controlling the signal STHB to keep black insertion pixel voltage to the pixels on the driven row; a backlight driving part for driving a plurality of backlight light sources arranged substantially in parallel to the rows of the pixels in such a manner that a plurality of pixels are sectioned into a plurality of groups; and an inverter control circuit.
The inverter control circuit controls a backlight driving part so as to start an action of sequentially lighting up and off the plurality of backlight light sources by synchronizing with the first start signal STHA at a prescribed duty ratio that corresponds to the ratio of the holding period of the gradation display pixel voltage to the holding period of the black insertion pixel voltage. The backlight driving part is configured with a plurality (m) of inverters that respectively generate driving voltages for the backlight light sources, and the inverter control circuit generates respective number (m) of pulse width modulation signals PWM (PWM1-PWMm) for controlling each of the inverters.
As shown in FIG. 75, the pulse width modulation signal PWM1 is generated by using the first and second start signals STHA and STHB among the control signals outputted from the vertical timing control circuit. The first start signal STHA is a reference timing for having the gradation display pixel voltage held at the OCB liquid crystal pixels of the first row, and the second start signal STHB is a reference timing for having the black insertion pixel voltage held at the OCB liquid crystal pixels PX on the first row.
However, there is a following issue with the scanning backlight drive of Patent Document 1.
That is, since there is a time lag in blinking of each of the backlight light source blocks, light emitted from a given light source block leaks to the other light source block areas. This causes an imperfect backlight light-off state.
FIG. 55-FIG. 57 show the light-off state with a light leakage between the light source blocks when a white display part is shifted, and show motion blur of the moving picture.
For example, as shown in FIG. 56, if there is no light leakage between the light source blocks and the lights are off completely, the motion blur of the moving pictures can be improved with respect to that of the normal drive. However, as shown in FIG. 57, if there is a light leakage between the light source blocks and light-off state is imperfect, the motion blur of the moving pictures are generated as ghost image. Thus, the quality of the moving pictures becomes deteriorated than that of the normal drive.
That is, with the scanning backlight drive, the backlight light-up timings in each of the divided light source blocks is different. Thus, the light-off state of the backlight becomes insufficient due to the light leakage occurred between the light source blocks. Therefore, the moving picture is viewed in a duplicated manner as in a case with ghost image, so that a sufficient moving picture improving effect cannot be brought out. In order to overcome such shortcomings, Patent Document 2 employs a structure to prevent the light leakage by providing the partition plates between the light source blocks.
However, the scanning backlight drive of Patent Document 2 has a following shortcoming, separately from those described above.
FIG. 58-FIG. 61 show moving pictures obtained by the line sequential drive and the scanning backlight drive of the liquid crystal display panel. As shown in FIG. 58, the liquid crystal display panel is driven with the line sequential scanning, so that frame information is rewritten sequentially from a scanning start line of the screen towards a scanning end line.
When moving pictures are displayed with the normal drive, eyes of human beings move while following the moving pictures. Thus, with a moving coordinate system of eyes of human beings, an image that is deformed from the information on the scanning start line in proportional to the moving speed of the moving picture is recognized as the image on the scanning end line, as shown in FIG. 60. The scanning lines of the liquid crystal display panel is fractionalized as described above, Thus, even though there is motion blur, deformation of the moving picture is sequential. Therefore, there is not so much uncomfortable feeling felt by viewers as a whole.
In the meantime, the dividing number of scanning backlight drive is naturally smaller than the scanning line number of a regular panel. It is because display scanning lines of a liquid crystal display panel are as many as four hundred eighty (480) for VGA (Video Graphics Array) and seven hundred sixty eight (768) for XGA (Extended VGA), while there are about several to twenty backlight light emitting areas due to restrictions set by the size of the light source such as CCFL or LED and the number of light-up circuits such as the inverters. Thus, it is not practical to provide the same number of light emitting areas as the number of scanning lines. Therefore, as shown in FIG. 61, with the scanning backlight drive, there are the moving image gaps generated at the boundaries of the light source blocks due to the light-up time differences in the divided light source blocks of the backlight.
Human beings have an advanced ability called “Vernier acuity” that is an ability to sensitively detect display with sudden discontinuous gap in image contours. Thus, for the display of the above-described case, eyes of human beings recognize even very small gap in a moving picture of a low speed, so that the picture quality is deteriorated.
Further, there is another shortcoming in the structure of Patent Document 2 in which the divided light source blocks are light-shielded structurally with the partition plates.
There are individual differences in the luminance and chromaticity of the light sources within the divided light source blocks of a normal backlight. As shown in FIG. 62 and FIG. 63, in a case of a normal backlight that does not employ the light-shielding structure of partition plates, the individual difference between the light sources are made mixed and in uniform to some extent. However, as shown in FIG. 64 and FIG. 65, in a case of the structure in which the light source blocks are light-shielded perfectly, the individual differences among the light sources are shown directly in the display. Thus, it becomes necessary to control the luminance by each light source block, and to select the light sources, for example. This results in increasing the cost.
As described above, when the light shielding structure (partition plates) are provided to avoid the light leakage between the areas, the moving pictures generate gaps on the boundaries of the divided areas of the backlight due to the light-up timing difference of the divided light source blocks, because the eyes of human beings follow the moving pictures. Therefore, the picture quality is more deteriorated than normal moving picture blurring.
Furthermore, the backlight structure becomes complicated, and the individual variations of the light sources such as the cold cathode tubes (CCFL) turn out directly as luminance unevenness of the divided light source blocks. Therefore, it is necessary to control the luminance of the light sources of each divided light source block, which results in a large increase in the cost.
Further, as shown in FIG. 64 and FIG. 65, there is such an issue in the light shielding structure that designing thereof is not easy because shadows of the light-shielding walls tend to appear on the screen, even though it is possible to prevent the light leakage.
Furthermore, FIG. 66-FIG. 72 show another shortcoming of the scanning backlight drive.
While the CCFL is the mainstream of the light source of the backlight, it has such a property that the optical response varies depending on the color wavelengths. FIG. 66-FIG. 71 show the optical responses of each color wavelengths (RGB) of CCFL. While the optical response of blue (B) is quick as in FIG. 68 and FIG. 71, the optical response of in the wavelength range of green (G) is slower as in FIG. 67 and FIG. 70. Thus, the light during the luminance response takes on the color. As a result, as shown in FIG. 72, the colored motion blur generated in the moving picture becomes changed and the picture quality is deteriorated if inexpensive CCFL is used for the light source in the scanning backlight drive.
As described above, the motion blur of the moving picture cannot be improved uniformly on the whole screen only with the scanning backlight drive.
Further, in the case of Patent Document 3, light-off of the backlight is started by the pulse width modulation signal PWM1 simultaneously with the start of writing the black insertion signal with the second start signal STHB, as in OFFSET of FIG. 75. That is, this technique improves the moving picture performance by simply synchronizing the timing of lighting-off the backlight with the timing of the black insertion signal, and there is no consideration over other factors such as gap in the moving pictures, ghost-image-like motion blur, and the like, which are caused due to various kinds of conditions (e.g., number of divided blocks of the backlight, relation between the light-off period and the black insertion period, transmittance response of the liquid crystal display panel, luminance response of the backlight light source, and the like).